1. Field of the Invention
The invention relates to methods of applying a wear-resistant layer to a surface of a downhole component for use in subsurface drilling.
2. Description of Related Art
The invention is applicable to downhole components of the kind which include at least one surface which, in use, engages the surface of the earthen formation surrounding the borehole. The invention relates particularly to rotary drill bits, for example of the drag-type kind having a leading face on which cutters are mounted and a peripheral gauge region for engagement with the surrounding walls of the borehole in use or of the rolling cutter kind. The invention will therefore be described with particular reference to polycrystalline diamond compact (PDC) drag-type and rolling cutter type drill bits, although it will be appreciated that it is also applicable to other downhole components having bearing surfaces. For example, bearing surfaces may be provided on downhole stabilizers, motor or turbine stabilizers, or modulated bias units for use in steerable rotary drilling systems, for example as described in British Patent No. 2289909. Such bias units include hinged paddles having bearing surfaces which engage the walls of the borehole in order to provide a lateral bias to the bottom hole assembly.
In all such cases the part of the downhole component providing the bearing surface is not normally formed from a material which is sufficiently wear-resistant to withstand prolonged abrasive engagement with the wall of the borehole and it is therefore necessary to render the bearing surface more wear-resistant. For example, the bodies of rotary drag-type and rolling cutter type drill bits are often machined from steel and it is therefore necessary to apply bearing elements to the gauge portion of such drill bit to ensure that the gauge is not subject to rapid wear through its engagement with the walls of the borehole. This is a particular problem with steel bodied drill bits where the gauge of the bit comprises a single surface extending substantially continuously around the whole periphery of the bit, for example as described in British Patent Specification No. 2326656.
One well known method of increasing the wear-resistance of the gauge of a drag-type or rolling cutter type drill bit is to form the gauge region with sockets in which harder bearing inserts are received. One common form of bearing insert comprises a circular stud of cemented tungsten carbide, the outer surface of which is substantially flush with the outer surface of the gauge. Smaller bodies of natural or synthetic diamond may be embedded in the stud, adjacent its outer surface. In this case the stud may comprise, instead of cemented tungsten carbide, a body of solid infiltrated tungsten carbide matrix material in which the smaller bodies of natural or synthetic diamond are embedded. Bearing inserts are also known using polycrystalline diamond compacts having their outer faces substantially flush with the gauge surface.
Another known method of increasing the wear-resistance of the gauge surface of a PDC drill bit is to cover the surface of the gauge, or a large proportion thereof, with arrays of rectangular tiles of tungsten carbide. Such tiles may be packed more closely over the surface of the gauge than is possible with bearing inserts, of the kind mentioned above, which must be received in sockets, and therefore allow a greater proportion of the area of the gauge surface to be covered with wear-resistant material at lesser cost. However, it would be desirable to use bearing elements which have greater wear-resistance than tungsten carbide tiles.
A known method for increasing the wear-resistance of the rolling cone cutter in rolling cutter bits is to include one or more rows of inserts on the gauge reaming portion of the rolling cutter. Typically, the inserts are cylindrical bodies which are interference-fitted into sockets formed on the gauge reaming surface of the rolling cutter, as shown in U.S. Patent Specification No. 5,671,817. The inserts may be formed of a very hard and wear and abrasion resistant grade of tungsten carbide, or may be tungsten carbide cylinders tipped with a layer of polycrystalline diamond. In addition, the gauge portion of each bit leg facing the borehole wall may be provided with welded-on hard facing and/or the same type of tungsten carbide cylinders are as fitted into the rolling cutters.
A material which is significantly more wear-resistant than tungsten carbide, and is also available in the form of rectangular blocks or tiles, is thermally stable polycrystalline diamond (TSP). As is well known, thermally stable polycrystalline diamond is a synthetic diamond material which lacks the cobalt which is normally present in the polycrystalline diamond layer of the two-layer compacts which are frequently used as cutting elements for rotary drag-type drill bits. The absence of cobalt from the polycrystalline diamond allows the material to be subjected to higher temperatures than the two-layer compacts without sufficient significant thermal degradation, and hence the material is commonly referred to as "thermally stable".
In one commercially available form of thermally stable polycrystalline diamond the product is manufactured by leaching the cobalt out of conventional non-thermally stable polycrystalline diamond. Alternatively the polycrystalline diamond may be manufactured by using silicon in place of cobalt during the high temperature, high pressure pressing stage of the manufacture of the product.
While TSP has the wear-resistance characteristics appropriate for a bearing element on a downhole component, it has hitherto been difficult to mount TSP on downhole components. Where blocks of TSP are to be used as cutting elements on drag-type drill bits it is necessary either to mold the bit body around the cutting elements, using a well-known powder metallurgy process, or to embed the blocks into bodies of less hard material which are then secured in sockets in the bit body. Where a bearing element is to be applied to a surface of a downhole component for the purpose of wear-resistance, however, it is preferable for the bearing element to be mounted on the surface of the component, particularly if the component is formed by machining, from steel or other metal, so that the bearing element cannot readily be embedded in the component. The present invention therefore sets out to provide novel methods for mounting TSP bearing elements on to a bearing surface of a downhole component.